In the manufacture of a hearing aid, it is necessary to create a housing containing components for the hearing aid wherein that housing comfortably fits within the ear canal.
Traditionally, a two-part silicon is injected by a technician into the ear canal/cavity where it hardens to create what is known as an impression. This impression is then removed from the patient's ear.
Thereafter, a so-called negative cast was created using clear silicon coated around the impression. This negative cast was then used as a quality tool and also used for adding external features to create what is known as an outer shell—the housing for the hearing aid components. These external features are to improve the retention of the final hearing aid in the ear cavity. These external features are made by applying a UV cured resin to the negative cast and to the shell.
More recently, it has been known to bypass the step of creating the negative cast to directly build the shell (hearing aid outer housing) by scanning the impression and creating a so-called point cloud (points around the outer peripheral surface of the impression). That point cloud was then used to create a so-called STL file. A STL file is a known file format native to the Stereolithography CAD software created by 3D Systems of Rock Hill, Carolina. This STL file is a detailed and modeled file which was then used in conjunction with such known Viper machine of 3D systems (also known as a Stereolithography Apparatus (SLA). The STL file is input into the machine and the machine then uses a UV-curable photopolymer resin and a UV laser to directly create the outer shell (component housing) as a hard material for the hearing aid without the use or creation of the negative cast. The stereolithography machine does this by building the part layer-by-layer from the resin and using a support material of the same resin to internally support the part being constructed by the SLA machine. This support material was like a scaffolding supporting the bottom of the shell and also around the shell, but not inside the shell. The support material on this Viper machine was always the same material as the shell and the support material were both hard materials. When the machine was finished processing the part, the support material was cut away and thus destroyed and discarded when exposing the internal shell.
A disadvantage of the above more recent method is that no negative cast is created or utilized. However, a negative cast is useful in serving as a quality control device so that when the final hearing aid housing (outer shell) is constructed, it could be fitted over the negative cast to determine whether the completed hearing aid housing (outer shell) was too loose, too tight, or had areas of insufficient clearance, all resulting in potential discomfort to the hearing aid user, or poor functionality of the hearing aid when it is placed in the ear canal. The negative cast is also used for checking cosmetic features of the final hearing aid.
With the earlier method where the negative cast was available, the shell (hearing aid housing) could be fitted onto the negative cast and one could check for undesirable rubbing or the too tight or too loose fit described above.